Abstract : Electrostatic probes placed in jet engine exhausts were found to display current signal pulses ('spikes') in an apparently random fashion. In some tests, where a failure of some engine component occurred, the frequency of the pulses was observed to increase by orders of magnitude prior to the failure. A tentative explanation attributed these pulses to impacts of engine distress-produced particulate matter on the probe. This phenomenon offers promise for the development of a device capable of alerting pilots to an imminent engine failure. The implementation of such a system, however, is not feasible without understanding the physical mechanisms linking the spikes to failure. This report is an evaluation of a considerable body of related experimental information. The study shows that most of the spikes are corona discharges between the probe and concentrations of ionic or particulate charges ('charged clouds'), convected with the exhaust gas.